In recent years, a method for producing compact sensor devices such as an acceleration sensor and a gyro sensor by using a micromachining technique has attracted lots of attention.
For example, Japanese Patent Early Publication No. 2005-292117 discloses a gyro sensor comprising a sensor unit formed by use of a silicon substrate, and first and second package substrates each formed by use of a glass substrate. In the sensor unit, a part of a sensing portion is formed on a movable portion, which is disposed inside of a frame portion. The first package substrate has a through-hole wiring electrically connected to the sensing portion. The first package substrate is bonded to a surface of the sensor unit over the entire circumference of the frame portion. The second package substrate is bonded to the other surface of the sensor unit over the entire circumference of the frame portion.
In this gyro sensor, however, since the sensor unit is bonded to the package substrates by means of anodic bonding, there is a problem that variations in sensor characteristics increase due to the influence of thermal stress resulting from a difference in linear expansion coefficient between silicon and glass. In addition, even when the package substrate is formed by use of the same silicon substrate as the sensor unit, the variations in sensor characteristics may occur due to the residual stress at a bonding portion formed by a high temperature bonding process such as reflow soldering.
Additionally, in a sensor device such as the gyro sensor described above where a part of the sensor unit is utilized as an electrode such as a movable electrode and a stationary electrode, a silicon substrate having a relatively smaller resistivity than the silicon substrate suitable for the formation of integrated circuits is used as the silicon substrate for the sensor unit. Thereby, the impedance component of the electrode can be reduced to achieve high performance. However, since the package substrate and the sensor unit easily work as an exogenous noise transmission channel, the noise component may be overlapped with output signals of the sensing portion. In this case, a reduction in resistance to electrical noise comes into problem.